1. Technical Field
The present invention concerns a rolling mill and a method of rolling round steel bars and steel wire using the mill.
2. Prior Art
As the rolling mills for producing round steel bars and steel wire there has been generally used two-roller type rolling mills, in which a pair of rollers oppositely disposed in parallel directions, and the peripheries of the rollers are pressed onto the materials to be rolled so as to roll and elongate them into predetermined sizes.
Such rolling steps using the two-roller type rolling mills are as illustrated in FIG. 7.
The Figure shows an example of round-oval type rolling. According to the conventional technology, as illustrated in the Figure, a material to be rolled 200 is sequentially rolled with rollers 202 to form alternative sections of oval-round-oval-round, and in the final finish-rolling step the material is shaped into round bars or wires of desired sizes.
The two-roller type rolling mills have a drawback that profiled rolls for every sizes are necessary because adjustable size ranges of each roller are small.
For example, in the case of producing a round steel bar of diameter 50 mm, a set of profiled rollers for this size is necessary. Also, in the case of producing a round steel bar of diameter 55 mm, another set of rollers having corresponding sizes is necessary. Thus, in order to produce round steel bars of various sizes many sets of rollers are necessary. Provision and maintenance of the roller sets are troublesome and expensive, and further, changes in the sizes of the steel bars to be produced necessitate set-up operation including change of roller sets. The set-up operation requires time and labor and lowers productivity of rolling operation.
On the other hand, there has been known four-roller type rolling mills, in which two pairs of rollers are installed at every 90.degree. axes disposition and the material is simultaneously rolled by the two pairs of rollers.
Further, it has been practiced to use two sets of the four-roller type rolling mills separately installed with 45.degree. rotated axes. The material to be rolled is firstly rolled down partly with the first four-roller type rolling mill and then, at downstream thereof, the remaining parts or free surfaces of the material is rolled down with the second four-roller type rolling mill.
In the rolling operation using the two sets of four-roller type rolling mills adjustable size ranges are wider, and thus, it is advantageous that round steel bars having different diameters, for instance, 50 mm and 55 mm, can be produced with the same set of roller pairs.
In the above described rolling operation it is necessary to use two sets of the four-roller type rolling mills of complicated structure and having individual driving sources. These factors necessarily require a higher investment. Guiding means to prevent deformation of the material being rolled are necessary between the first and the second four-roller type rolling mills. Also, because the first and the second four-roller type rolling mills are mutually independent, matching of rotating speeds of the rollers of the first and the second four-roller type rolling mills is required. Otherwise, tension and/or compression will be effected to the material being rolled and it will be difficult to carry out desired rolling.